With development of electronic communication industry, a portable electronic device is essential in modern life and becomes an important means of delivering information. In recent years, the electronic device has been developed in various forms worn on a body of a user to improve portability and accessibility of the user. For example, the electronic device may be a wireless audio output device (e.g., a wireless earphone or a wireless headphone) that is worn on an ear of the user or contacts the ear of the user. For example, the wireless audio output device may include a pair of output devices, which are respectively inserted into both ears of the user and which are coupled to each other wirelessly.
Each of the pair of output devices may include a battery. Even though the wireless audio output devices are used after the batteries of the wireless audio output devices are fully charged, each of battery levels of the pair of the output devices may be different. Accordingly, a battery of one of the pair of output devices may be discharged first, and then usability of the wireless audio output device may be reduced.